1. Field of the Invention
This invention relates generally to enhanced oil recovery (EOR) techniques and, more particularly, for such techniques for improving the production economies and recovery efficiency of hydrocarbons from a reservoir after primary reservoir depletion has occurred.
2. Brief Description of the Prior Art
The present invention will be described with respect to enhanced oil recovery (EOR) techniques based on water flooding of a producing reservoir. It will be understood by those skilled in the art, that any or all EOR projects that inject fluids of any type for the purpose of increasing the recovery of hydrocarbon can be benefitted by the use of the concepts of the present invention. Similarly, the present invention will be described with respect to a system for EOR in which a deeper producing zone having contrasting permeability has recovery enhanced by the techniques of the present invention, but it will be understood by those of skill in the art that such techniques could also be employed to enhance production in shallower production zones having a contrasting permeability to the primary producing zone, or in both shallower and deeper production zones having a contrasting permeability than the primary producing zone, if desired. Moreover, while the present invention is described with respect to a particular injection/recovery well geometry, it will be appreciated by those skilled in the art that it could also be useful in other injection/recovery well geometries than those described herein.
In prior art water flooding techniques a pattern or geometrical arrangement of producing wells and injection wells are drilled into a producing formation. Water is pumped into the production interval from the water injection wells and "sweeps" the formation fluid toward the production wells where the formation fluid is produced to the surface by conventional lifting means, such as a surface driven pump, a submersible pump, a gas lift arrangement or the like. A particular geometry for such an operation could be the "five spot" geometry in which the injection wells are approximately spaced on the corners of a square of the desired dimension, as determined by formation permeability, fluid viscosity, etc., and the producing well is located at the center of the square. The injected water in the producing formation forces the formation fluids to move toward the producing well where it is pumped to the surface, separated into oil and water components, and the produced water is re-injected or carried away to be disposed of in other manners.
In such techniques, if the producing interval contains other zones having higher permeability and are not isolated from other nearby zones having higher permeability, then the injected water can preferentially go into such higher permeability zones and be "stolen" by such "thief zones." Problems of this type or "channeling" of injected fluids along the well bore into higher permeability zones or other zones due to improper cementing of casing or failed completion techniques can also result in the water being injected going into undesired permeable zones and not stimulating EOR recovery in the desired producing zone. Also in the prior art water flood recovery processes, as recovery proceeds, the water cut of the produced fluid increases. It is not uncommon in mature fields to produce fluid having 95% or greater water cut using these techniques. Of course, lifting 95% undesired fluid to the surface is not an efficient use of lifting processes.